The function of stone carrying in the black wheatear, Oenanthe leucura

Abstract

Abstract. Mated pairs of the black wheatear, but particularly the males, carry large numbers of relatively heavy (3-9 g on average) stones in flight to nest cavities and other sites in caves and cliffs during the prelaying period of each clutch. The 40-g birds carry on average 1-2 kg of stones during periods of 2-22 days (average 7·8), and stone carrying is therefore costly in terms of time and energy. Five functional explanations for the maintenance of this behaviour are considered. (1) The nest support hypothesis, that stones form a solid base for the nest, is refuted because large horizontal nest cavities with no need for nest support contained as many stones as those in other sites, nests were frequently situated in sites without stones, and stones were carried to sites not subsequently used for nests. (2) The thermoregulation hypothesis, that stones may moderate large diel temperature fluctuations and thus reduce the cost of incubation and brooding, is refuted because nests were frequently situated in sites without stones, there were as many stones in caves where temperatures fluctuated little as in natural cavities where temperatures varied widely, hatching success was unrelated to the number of stones and the cooling rate of nest contents was not affected by the presence of stones. (3) The weather protection hypothesis, that a large number of stones may protect the nest from wind, rain, or dust, is refuted because nests were frequently situated in sites without stones, and no case of nest failure owing to inclement weather was recorded. (4) The anti-predation hypothesis, that stones may prevent predation of nest contents or incubating females, is also refuted because stone carrying prevented use of relatively safe nest sites above 3·5 m, and nests suffering from predation had as many stones as successful nests. (5) The sexual display hypothesis, that pair members use the stone-carrying display to assess the quality of the partner and adjust their relative parental investment accordingly, does account for the behaviour since clutch size was positively and laying date negatively correlated with the number of stones carried to cavities other than nest sites, and the seasonal production of young was positively associated with the seasonal number of stones carried to cavities other than nest sites. The number of stones carried to nest sites had less predictive capacity. Stone carrying was also associated with the extent of paternal care. The behaviour showed a relatively high repeatability within and between seasons, implying strong and consistent differences between individuals. Stone carrying in the black wheatear can therefore be considered a post-mating but prenesting sexual display which allows adjustment of reproductive effort by females to the parental and/or phenotypic quality of partners.